1. Field of the Invention
This invention relates to an exposing apparatus used for exposing photoresist films to light in fabrication of printed circuit boards and particularly to an exposing apparatus of the type in which the interface between the original and a material to be exposed is evacuated during exposure to ensure intimate contact therebetween.
2. Description of the Prior Art
For the positioning of the original and the material to be exposed, mechanical methods including the use of a pin or the like have been commonly used. However, in recent years, with the increasing pattern density of printed circuit boards, the mechanical methods described above have proved to be incapable of providing the required accuracy. Thus, a system is being increasingly used which employs an optical sensor, such as a CCD camera, to read marked positions associated with the original and the material to be exposed, so as to effect the positioning of the original and the material.
FIG. 5 shows a conventional exposure device using said CCD camera. In FIG. 5, an original 1 which is to become a photomask is held stuck to the lower surface of a transparent original holding plate 2, which is fixed to an original frame 3. The original frame 3 is made vertically movable, as indicated by double arrows 5, by a suitable number of vertical guide and air cylinder assemblies 4. A suitable number of CCD cameras 6 are disposed above the original 1.
A material to be exposed 7 which is to become a printed circuit board is placed on a material holding table 8. The holding table 8 is formed with a vacuum passage 9, which is connected to a vacuum source (not shown) through a valve 10. The vacuum passage 9 communicates with a plurality of suction holes 11 having their openings distributed in the upper surface of the holding table 8. The material to be exposed 7 is fixed on the holding table 8 in that a vacuum is imparted to each suction hole 11 through the vacuum passage 9.
The holding table 8 is connected to an X-Y-.theta. unit 12. The unit 12 is adapted to be actuated when the positioning of the original 1 and the material to be exposed 7 is to be effected, said unit 12 being designed to move the holding plate 8 in the X-direction and the Y-direction orthogonal thereto and rotate it, in the plane of the main surface of the material to be exposed 7.
A packing 13 of elastic material having a V-shaped cross-section is attached around the periphery of the holding table 8. The packing 13 is capable of contacting the aforesaid original holding plate 2. During the time the packing 13 is in contact with the original holding plate 2, the space 14 surrounded by the packing 13 is in the sealed state. To impart a vacuum to such space 14, the holding table 8 is formed with a vacuum passage 15, to which a vacuum source (not shown) is connected through a valve 16.
In the exposing apparatus shown in FIG. 5, to effect the positioning of the original 1 and the material 7, first, respective marks (not shown) applied to the original 1 and material 7 are read by the CCD cameras 6 and relative positional deviations of these marks are calculated by a computer (not shown). And according to the result of the calculation, the X-Y-.theta. unit 12 is actuated to move the material 7 through the holding table 8 in the X, Y and .theta. directions to achieve the positioning. When such positioning is to be effected, it is desirable that the original 1 and the material 7 be either superposed with a slight clearance (0.02-0.05 mm) therebetween or lightly contacted with each other.
When the positioning described above is completed, ultraviolet rays 17, for example, indicated by arrows are radiated. At this time, it is desirable that the original 1 and the material 7 be intimately contacted. For this reason, a vacuum is imparted to the space 14 via the vacuum passage 15.
In the above exposure operation, the function or behavior of the packing 13 will now be considered.
The packing 13 is formed of elastic material as described above and, as shown in FIG. 5, since the space 14 has to be sealed when the original holding plate 2 is lowered to the extent that at least the original 1 and the material 7 come in contact with each other, it is undesirable to make the arrangement so that it is not until the original 1 and the material 7 come in contact with each other that the packing 13 comes in contact with the original holding plate 2. In other words, the packing 13 has to be designed so that it begins to contact the original holding plate 2 in the course of downward movement of the original holding plate 2.
As can be seen from such function or behavior of the packing 13, when the positioning of the original 1 and the material 7 is to be effected, even if there is a slight clearance therebetween it follows that the packing 13 has already been kept in contact with the original holding plate 2. For this reason, the contact of the packing 13 with the original holding plate 2 produces a substantial resistance when the material holding table 8 is moved in the X, Y and .theta. directions. Such resistance constitutes a serious obstacle to moving the original 1 and the material 7 relative to each other with high accuracy in the X, Y and .theta. directions in an effort to effect the positioning thereof.